Answer:
2304
Step-by-step explanation:
a1 = 9
a2 = 4*a1 = 4*9 = 36
a3 = 4*a2 = 4*4*a1 = 4*4*9 = 144
a4 = 4*a3 = ... = 4*4*4*9 = 576
a5 = 4*a4 = ... = 4*4*4*4*9 = 2304
in other words
an = a1*4^(n-1)
you have to run through the calculation (in our case a simple multiplication by 4) n-1 times with the starting value of a1.
so, a5 = 9 * 4^4
for example a20 would then be
a20 = 9 * 4^19
